1. Field of the Invention
The present invention relates to a method of forming a film used in a semiconductor device such as a semiconductor integrated circuit device or a display device such as a liquid crystal display device. The present invention also relates to a method of manufacturing a semiconductor device such as a thin film transistor (TFT) or a metal oxide semiconductor device (MOS device) and to a semiconductor device. The present invention further relates to a method of manufacturing a display device such as a liquid crystal display device, an organic EL display device or an inorganic EL display device, and to a display device.
2. Description of the Related Art
In general, a silicon oxide film is used as a gate insulating film in a semiconductor device such as a thin film transistor (TFT). A plasma CVD (plasma enhanced chemical vapor deposition) method is known to the art as a method of forming a silicon oxide film under temperatures not higher than 600° C. so as to prevent adverse effects on the substrate.
In the plasma CVD method, a silicon oxide film is formed as follows. In the first step, a monosilane gas is mixed with an oxygen gas, followed by supplying the mixed gas into a chamber in which a substrate is arranged. Under this condition, a plasma is generated within the chamber so as to achieve the plasma discharge of the monosilane gas and the oxygen gas, thereby depositing silicon oxide on the substrate.
The conventional plasma CVD method gives rise to the problem that the oxygen atoms are not supplied sufficiently, with the result that formed is a silicon oxide film having a large oxygen deficiency. Naturally, it is of high importance to overcome the problem.
Also, proposed in patent document e.g., Japanese Patent Disclosure (Kokai) No. 11-279773, is a plasma CVD method using a mixed gas consisting of two kinds of gases including gaseous molecules and a rare gas having appropriate excitation levels relative to the gaseous molecules.
It should be noted that, in a top gate type TFT used in a display device, silicon oxide is deposited in general by a plasma CVD method on a semiconductor layer processed in the form of an island and having a thickness of about 50 nm so as to form a gate insulating film having a thickness of 80 to 100 nm.
The scale of the display device has been enlarged, and the display device has been made to perform many functions. In this connection, the TFT has come to be applied to a new display device such as an organic EL display device. Such being the situation, miniaturization of the TFT has come to be required while improving the device characteristics of the TFT. In order to miniaturize the TFT, the gate insulating film is required to be rendered thinner. To be more specific, when it comes to a TFT having a channel length of 1 nm, it is required for the thickness of the gate insulating film to be decreased to 30 nm.
When it comes to a top gate type TFT in which a gate insulating film is formed on a semiconductor layer formed in the shape of an island, it is necessary for the gate insulating film to be formed in a manner to cover the entire region of the semiconductor layer including the stepped portion formed in the semi-conductor layer. It follows that the current leakage through the gate insulating film tends to be increased in the stepped portion. It should be also noted that the amount of leak current will increase of the gate insulating if the gate insulating film is made of an oxide silicon film that is as thin as 30 nm.
One approach to solve the above problems is to use a laminated structure of plasma CVD films, as seen in the documents 1 and 2 set forth below.
According to the technology disclosed in document 1 referred to above, it is possible to form a film under the temperature lower than that required in the organometallic gaseous phase growth method while suppressing the damage done to the underlayer. In addition, the film can be formed at a film-forming rate higher than that for the atomic layer depositing method. However, the zirconium oxide film formed by the technology disclosed in document 1 gives rise to the problem that the oxygen deficiency in the formed film is significantly large.
Document 1: M. Goto, et al., “Surface Wave Plasma Oxidation at Low Temperature for Gate Insulator of Poly-Si TFTs”, Dec. 4-6, 2002, [Proceedings of The Ninth International Display Workshops], p 355 to p 358.
Document 2: “Formation of Zirconium Oxide Film having High Dielectric Constant by Plasma CVD using Organometallic Material as Precursor” by Reiji Morioka, et al., p 317 to p 318 of the collection of lecture documents of the “20th Plasma Processing Research Meeting” held on Jan. 29, 2003 and sponsored by Plasma Electronics Branch of Applied Physics Institute (an incorporated body).
As described above, if the thickness of the gate insulating film is decreased to about 30 nm, it is difficult to obtain sufficient device characteristics. In other words, the decrease in the thickness of the silicon oxide film is limited. Such being the situation, the metal oxides having a dielectric constant higher than that of silicon oxide such as hafnium oxide and zirconium oxide have come to attract attention as a material of the gate insulating film. In other words, in the case of using a metal oxide having a high dielectric constant as a material of a gate insulating film, it is expected that the thickness of the gate insulating film can be further decreased while maintaining the capacitance of the gate insulating film equal to that of the gate insulating film formed of a silicon oxide film.
An organometallic gaseous phase growth method (MOCVD method), a sputtering method or an atomic layer depositing method (Atomic Layer Deposition: ALD) as a deposition method of a very thin film are known to the art as a method of forming a film made of a metal oxide such as hafnium oxide or zirconium oxide.
In the organometallic gaseous phase growth method, a film is grown by decomposing an organometallic compound gas used as a raw material by using the substrate heated to 500° C. to 700° C., with the result that it is difficult to form a metal oxide film on a general type of glass substrate or a plastic substrate.
A film can be formed at a relatively low temperature in the case of employing the sputtering method. However, since particles running at a high speed collide against the substrate in the case of employing the sputtering method, the underlayer film tends to be damaged. It follows that the metal oxide film formed by the sputtering method has a high interface state density and, in addition, involves a significant oxygen deficiency. Incidentally, in order to make up for the oxygen deficiency in the metal oxide film, it is necessary to employ, for example, a plasma processing or an annealing treatment under high temperatures after the film formation. It follows that the number of manufacturing processes needed in the formation of the metal oxide film is increased, which is disadvantageous.
In the atomic layer depositing method, the atomic layers are deposited one layer at a time and, thus, the film-forming rate is very low. It follows that the atomic layer deposition method is not adapted for the formation of a TFT because it is necessary for the gate insulating film included in the TFT to have a thickness of tens of nanometers.
A film-forming method employing a plasma CVD technology using an organometallic material as a precursor is proposed as another method of forming a metal oxide film. The particular film-forming method is summarized below.
In the first step, tetrapropoxy zirconium (Zr(OC3H7)4) is mixed with an oxygen gas and an argon gas. The ratio of the oxygen gas to the argon gas within the mixed gas is 1:5. In other words, the percentage of the partial pressure of the argon gas based on the total pressure of the mixed gas is 80%. Then, the mixed gas is introduced into a chamber in which a substrate is arranged. Under this condition, a plasma is generated within the chamber so as to achieve a plasma discharge of the tetrapropoxy zirconium and the oxygen gas, thereby depositing zirconium oxide on the substrate.
In the technology disclosed in document 1 referred to previously two kinds of gases consisting of gaseous molecules and a rare gas having an appropriate excitation state relative to the gaseous molecules are mixed so as to permit the rare gas to decompose the gaseous molecules into an atomic state. In other words, in the case of forming a silicon oxide film, a monosilane gas is mixed with an argon gas so as to generate the atomic silicon and, at the same time, an oxygen gas is mixed with a xenon gas so as to generate the atomic oxygen. It follows that, in the technology disclosed in document 1, at least two plasma generating apparatuses are required for forming a silicon oxide film or a metal oxide film. Such being the situation, the manufacturing apparatus is rendered complex and the manufacturing price is increased. In addition, the technology disclosed in patent document 1 gives rise to the problem that it is impossible to use a gas consisting of an organic silicon compound such as tetraethoxy silane (TEOS) as gaseous molecules for generating the silicon atoms.
Further, document 2 pointed out above refers to, for example, the relationship between the Kr dilution ratio and the film thickness owing to the plasma oxidation and to the relationship between the microwave output and the oxygen atom density. However, document 2 simply refers to the technology of performing a surface wave plasma oxidation at a low temperature for forming a gate insulating film for a TFT, failing to refer to, for example, the film-forming technology as in the present invention.